The present invention relates generally to fluid filters and more particularly to a filter which has distinctive inlet apertures defined, in part, by angularly declining surfaces. This invention further relates particularly to methods for making such filters. Filters of this invention present a distinct advantage in extracorporeal blood systems.
Fluid filters come in many shapes and sizes depending primarily on their intended usage. For example, filters positioned at the inlet to a pipe or tubing system are often also tubular in form. Conical and tapered cylindrical forms are alternatives used in these situations as well. Aperture size and quantity may also drive the ultimate geometric shape chosen for use. So too may the fluid to be filtered.
An example of a fluid system having special requirements which can be significantly impacted by filter shape is a blood flow system outside the body, i.e., an extracorporeal blood system. Poor flow patterns can create problems for blood flowing through an extracorporeal blood system. Excessively turbulent or unduly slowed or stagnated blood flow may activate the blood""s clotting processes causing the formation of blood clots and strands in the blood. Clotting of blood in an extracorporeal blood system may result in occlusion of the tubing lines or in injury to the patient.
Extracorporeal blood systems usually include devices commonly known as drip chambers or bubble traps (referred to herein generally as bubble traps). One purpose of such devices is to capture and remove potentially harmful elements (such as air bubbles or blood clots) from the blood prior to treating the blood or returning it to the patient. Filters are often used to assist in this removal, particularly for catching blood clots and other particulates. A typical filter of this type is a conical or cylindrical device having numerous perforations formed therein. Examples of such filters can be found used in the bubble traps disclosed in the U.S. Patents to Brugger (U.S. Pat. Nos. 5,503,801 and 5,591,251) and Raabe et al. (U.S. Pat. No. 5,489,385). A bubble trap with a filter of this type is also commonly incorporated into and thereby forms a distinct part of a cartridge cassette of an extracorporeal blood tubing and cartridge set such as those disclosed in the U.S. Patents to Heath et al. (U.S. Pat. Nos. 4,666,598 and 4,770,787), inter alia.
It is believed that numerous filter designs may promote blood clotting because, among other possible detriments, they present an inherent resistance to ordinary blood flow. Resistance in a blood flow path slows the blood""s progress which, in turn, allows the blood to initiate clotting. Such clotting can occur at points of stagnation or along surfaces presenting a high degree of frictional resistance. Resistance may also cause turbulence in blood flow filters and is thus sought to be avoided here as well. Turbulent blood flow may lead to clotting or the formation of air bubbles in the blood. If returned to the patient in the blood, either of these present a risk of adverse health consequences to the patient.
Though still operable for passing blood, filters of the prior art nevertheless suffer geometric inefficiencies which present friction, stagnation and/or general slowing of the blood flow; any of which conditions possibly leading to blood clotting. For instance, the filters in the bubble traps of Brugger ""801 and ""251 (referred to above) present a plurality of substantially square, right-angled entry windows (in cross-section) which force the generally downwardly flowing blood in those bubble traps to make right-angled turns in order to enter the respective filters. Such right-angled turns can have the effect of slowing the blood flow as well as creating stagnation points at the lines of flow divergence. The filter designs of Raabe et al. ""385 suffer similar geometrical, flow-impairing drawbacks. The filters shown in Raabe have external projections which also force substantially right angled flow redirections for entry of blood into those filters. It is thought that such redirections cause slowing and stagnation which may promote clotting and thereby, flow restrictions.
Thus, it is apparent that there remains a distinct need for continued improvements in blood flow filters which effectively remove solids from the blood yet provide unhindered passage of the blood therethrough. It is toward this end that the present invention is directed.
The filter of the present invention has a substantially cylindrical or slightly tapered body design which has a plurality of elongated apertures formed therethrough. Each aperture is defined side to side by a pair of elongated, wedge-like rib members disposed one on each side of each aperture, and top to bottom by a pair of cross members which present a substantially square top side and an angularly declining bottom side. The end effect is a substantially rectangular entry aperture for fluid flow with the exception of the declining bottom side which promotes smooth, low resistance entry flow.
The present invention is also directed to a method of manufacture of filters having the above-described characteristics. In particular, the preferred method involves an injection molding process in which a molten plastic (such as high-density polyethylene, HDPE) is injected into a two-part mold. The mold generally comprises a smooth, substantially cylindrical cavity and a notched and grooved core which is inserted into the cavity in a partial surface to surface sealing contact relationship to complete the filter mold. Due to the extremely close tolerances required by a such a core and cavity surface to surface sealing relationship, unique mold alignment modifications were also developed.
These and other features of the present invention will be further illuminated in the following detailed description read in conjunction with the accompanying drawings which are described briefly below.